Method for blow-molding hollow container and blow air cylinder

ABSTRACT

A blow molding method by horizontal blow molding of piercing the wall of a parison having been extruded by an extruder with a blow nozzle is disclosed, which comprises moving forward the blow nozzle to pierce the parison, introducing blow air at least in the direction toward the bottom, i.e., opposite to the bottle neck and, after shaping of a hollow container by the blow air, forming an opening in the parison at a position in the vicinity of or above the blow nozzle-pierced portion to discharge the blow air having circulated inside the hollow container. In this blow mollding method, there is preferably used a blow air cylinder, which is constituted by (A) a blow nozzle having a mechanism for registering blowing inlets in at least part of a portion freely sliding forward and backward inside the body, having a tip portion with an outer diameter smaller than at about the center portion thereof and being of a shape of a closed injection needle, and having the blowng inlets located slightly at the back of the tip portion where the outer diameter is larger than that at the tip portion and smaller than that at the central portion and capable of introducng blow air in the direction toward the bottom, i.e., opposite to the bottle neck in an amount of 30% or more of the total blow air amount; and (B) a body provided outside and, at least partly, in cntact with the blow nozzle.

This application is a continuation of application Ser. No. 08/244,901filed Sep. 14, 1994 now abandoned.

TECHNICAL FIELD

This invention relates to a method for blow-molding a hollow containerand to a blow air cylinder and, more particularly, to a blow-moldingmethod which enables one to shorten the time for cooling moldingsimmediately after being blow-molded. This invention also relates toproducing moldings having good dimensional accuracy. A blow air cylinderis taught which is particularly adapted for the blowing method.

PRIOR ART

In molding hollow container from a thermoplastic resin according to blowmolding technique, it has been well known to pierce the upper portion ofa parison having been melt extruded through a die head of an extruderwith a blow nozzle for introducing blow air, and blow a compressed airinto the parison through the nozzle to thereby press the parison againstthe wall of a metal mold for shaping and cooling.

The blow nozzle for use in the blow molding is a member which, upon blowmolding, pierces the upper wall of the parison and which functions tointroduce a compressed air through a blowing inlet or inlets provided atthe tip of the nozzle or in the vicinity thereof, and has asophisticatedly designed structure.

For example, Japanese Examined Utility Model No. S52-19033 describes thestructure of a blow molding nozzle which has a pin with a sharp tip,said pin having a passageway for blowing air formed at its centerextending in the direction of central axis and pores connecting to thepassageway almost at a right angle therewith in the vicinity of the tipof the pin and having one or more linear grooves outside between the tipand the pores.

The form of the nozzle tip for blow molding must be so designed as tofacilitate piercing of the wall of parison having been extruded in aheated state with the nozzle and subsequent smooth blowing. The form ofthe blowing inlets of the above-described utility model, too, hasgrooved blowing inlets toward the tip from this viewpoint.

Japanese Unexamined Utility Model No. 57-169510 discloses the structureof a blow molding nozzle wherein the tip is of a shape of closedinjection needle with an angular cut of 10 to 45 degrees and which hastwo air-blowing holes formed on the way to the tip at a right angle witha blow air passageway. In this utility model, too, the nozzle tip has aspecific structure which facilitates piercing of the wall of a parisonwith even a thick nozzle.

In a horizontally blow molding method, a blow nozle pierces the wall ofa parison having been heat-extruded into a metal mold and, after closingthe metal mold, a compressed air (blow air) is introduced into theparison to completely press the soft-state parison against the insidesurface of the metal mold for shaping and cooling, thus one blowing stepbeing completed.

PROBLEMS THAT THE INVENTION IS TO SOLVE

In the conventional blow molding methods using such known blow nozzles,however, blow air introduced through the blowing inlets fills up thebottle at the time when the soft-state parison is pressed against thewall of the metal mold and, therefore, flow of the blow air isdiscontinued. Thus, subsequent cooling of the blown bottle is mainlyconducted by circulating cooling water in the metal mold in contact withthe bottle.

However, this method requires a considerably long time for cooling theshaped bottle still kept at an elevated temperature to such a degreethat the metal mold can be opened. Thus, there arises a problem in viewof moldng efficiency and, in addition, there arises another technicalproblem that, since the bottle is not uniform in thickness, uniformcooling can not be attained.

That is, there are known no blow nozzles of the structure designed forconducting the step of cooling moldings (bottles) most effectively,though nozzle structures suited for piercing the wall of a parison orfor introducing the compressed air have been known.

OBJECTS OF THE INVENTION

An object of the present invention is to provide a process for blowmolding hollow container having no molding deformation and havingexcellent quality, in which the blow air introduced into the container(bottle) well circulate therein even after shaping of the bottle torapidly cool the bottle.

Another object of the present invention is to provide a blow aircylinder adapted for the method.

DISCLOSURE OF THE INVENTION

The present invention is proposed for attaining the above-describedobjects and relates to a method for blow molding hollow container usinga blow nozzle of a specific structure and to a blow air cylindercontaining the nozzle.

That is, according to one aspect of the present invention, there isprovided a method for blow molding hollow container by horizontal blowmolding of piercing the wall of a parison having been extruded through adie head of an extruder with a blow nozzle, which comprises introducingblow air at least in the downward direction, i.e., in the directionopposite to the bottle neck, to form an opening.

According to another aspect of the present invention, there is provideda blow molding method, in which a part of a parison in the vicinity ofor above the blow nozzle-piercing portion is thinned so much, aftershaping of the hollow container, that the part is broken to form anopening through which the blow air having circulated inside the hollowcontainer is discharged.

According to a further aspect of the present invention, there isprovided a blow molding method, in which the amount of air blown in thedirection toward the bottom, i.e., opposite to the bottle neck, islarger than that blown in other directions.

According to a further aspect of the present invention, there isprovided a blow molding method, in which blowing inlets are formed inthe vertical direction, with the amount of air blown in the directiontoward the bottom, i.e., opposite to the bottle neck, being larger.

According to a further aspect of the present invention, there isprovided a blow molding method, in which blowing inlets are formed inthe vertical and horizontal directions, with the amount of air blown inthe direction toward the bottom, i.e., opposite to the bottle neck,being larger than that in each of the other three directions.

According to a still further aspect of the present invention, there isprovided a blow air cylinder, which is constituted by (A) a blow nozzlehaving a mechanism for positioning blowing inlets in at least part of aportion freely sliding forward and backward inside the body, having atip portion with an outer diameter smaller than at about the centerportion thereof and being of a shape of a closed injection needle, andhaving the blowng inlets located slightly at the back of the tip portionwhere the outer diameter is larger than that at the tip portion andsmaller than that at the central portion and capable of introducng blowair in the direction toward the bottom, i.e., opposite to the bottleneck, in an amount of 30% or more of the total blow air amount; and (B)a body provided outside and, at least partly, in contact with the blownozzle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating the state of a blow nozzle usedin the prior art blow molding method.

FIG. 2 is a schematic view illustrating the state of a blow nozzle usedin the blow molding method of the present invention.

FIG. 3 is a cross-sectional side view of a blow nozzle used in the priorart blow molding method.

FIG. 4 is a cross-sectional side view of a blow nozzle used in the blowmolding method of the present invention.

FIG. 5 is a cross-sectional front view showing one embodiment of theblow nozzle to be used for the blow molding method of the presentinvention.

FIG. 6 is a cross-sectional view of the blow air cylinder of the presentinvention before work.

FIG. 7 is a cross-sectional view of the blow air cylinder of the presentinvention in the state of initiating introduction of a blow air formoving forward the blow nozzle of the blow air cylinder.

FIG. 8 is a cross-sectional view of the blow air cylinder of the presentinvention in the state of the blow nozzle being moved forward by thepressure of the blow air introduced thereinto.

FIG. 9 is a cross-sectional view of the blow air cylinder of the presentinvention in the state of initiating introduction of a blow air formoving backward the blow nozzle.

FIG. 10 is a cross-sectional view of the blow air cylinder of thepresent invention in the state of being restored to the initial positionby the pressure of the air.

BEST MODE FOR PRACTICING THE INVENTION

As is described hereinbefore, a first technical feature of the presentinvention is to form blowing inlets of the blow nozzle so that theamount of blow air in the downward direction, i.e., in the directionopposite to the bottle neck, be at least 30%, preferably at least 50%,more preferably at least 70%, of the whole blow air amount.

The present invention has been described referring to the case where theparison is held with the bottle neck directed upward. However, it caneasily be understood that, where the parison is held with the bottleneck directed downward, the blow air should be introduced in the upwarddirection.

Another technical feature of the present invention is that, aftershaping of a bottle by filling the inside of a metal mold with the blowair, part of the parison in the vicinity of or above the blownozzle-pierced portion be thinned and broken to form an opening throughwhich blow air having circulated in the interior of the bottle isdischarged.

That is, the opening is formed, after shaping of the bottle, by thepressure of the blow air, and this opening serves to adjust discharge ofthe blow air filling the bottle. Hence, the blow air does not stagnatein the metal mold after shaping of the bottle but circulates within thebottle to rapidly cool the hot bottle.

Combination of the above-described two technical features enables one tomarkedly effectively cool the bottle.

To describe this in more detail, the blow nozzle to be used in thisinvention allows most of the introduced blow air to rapidly proceeddownward to the bottom of the bottle due to the specific structurethereof as has been described hereinbefore. When the blow air reachesthe bottom of the bottle, the parison in a softened state is instantlyshaped to the form of the metal mold and, at this point after shaping ofthe bottle, part of the wall of parison in the vicinity of or above thenozzle-pierced portion is thinned and broken to form an opening, thusthe blow air circulating inside of the bottle to effectively cool it.The blow air filling the metal mold is discharged through the formedopening to ensure subsequent circulation of blow air.

Thinning of part of the parison wall is conducted by forming in acorresponding portion an opening communicating with outside of the metalmold, and pressing the softened parison against the opening by thepressure of blow air to form a thin concavity. The thinned portion isbroken by further introducing blow air.

Alternatively, the above-described opening may be formed by piercing thewall of said parison with a needle-like member from outside. Theneedle-like member has a tip portion of injection needle form which maybe hollow or solid. That is, as to a hollow needle-like member, itsuffices to pierce the parison wall with it because blow air isdischarged through the hollow portion and, as to a solid needle-likemember, an opening for discharging the blow air is formed by piercingthe wall of a parison with the needle and rapidly drawing it out.

In blow molding bottles, the pinch-off portion at the bottom is formedin a thickness slightly thicker than other portions. If the bottomportion is not sufficiently cooled, there result moldings withdeteriorated dimensional accuracy.

According to the present invention, however, the pinch-off portionhaving a greater thickness is extremely effectively cooled and thereresult bottles with excellent dimensional accuracy in a short time,because a large amount of blow air is introduced in the direction towardthe botom, i.e., opposite to the botle neck, through the blow nozzle,and the blow air curculates inside the bottle and is then dischargedthrough an opening formed in part of the parison by the pressure of theblow air.

As has been described hereinbefore, the blow nozzle used in the presentinvention must be so constructed as to allow the blow air to beintroduced into a parison in at least the direction toward the bottom,i.e., opposite to the bottle neck.

That is, the blow nozzle to be used in the blow molding method of thepresent invention must have a blowing inlet or inlets facing the bottom.The nozzle may have one blowing inlet facing the bottom, or may haveadditional inlets facing any direction. In forming a plurality ofblowing inlets, they must be constituted so that blow air can beintroduced in more amount through the blowing inlet facing the bottomthan that through other inlets.

Specifically, the amount of blow air introduced in the direction towardthe bottom is more than 30%, preferably more than 50%, and morepreferably more than 70%, of the whole blow air.

In the present invention, "direction toward the bottom" means not onlythe direction right under or above the nozzle but obliquely downward orupward directions in which the blow air is introduced against the insideof the bottle body.

In the conventional blow molding method, a blow air is introducedthrough the upper portion of a parison (in the case of holding theparison with its bottom directed downward) using a blow nozzle as shownin FIG. 1, and hence most of the blow air is once blown against the wallopposite to the blowing inlet, then circulates in both downward andupward directions. However, since the amount of the blow air in thedownward direction is the same as that in the upward direction, blow airnot having circulated the inside of the bottle will be dischargedtogether with blow air having circulated the inside if an opening isformed by breaking upper part of the parison by the pressure of blowair, thus the amount of blow air circulating downward becoming extremelysmall.

In addition to the blow nozzle shown in FIG. 3, there have also beenknown those nozzles in which blowing inlets are formed on the way to thetip portion in two directions at a right angle with a blow airpassageway, as disclosed in the foregoing Japanese Unexamined UtilityModel Publication No. S57-169510. In this type of nozzles, too, blow airis introduced into a parison merely in the upward and downwarddirections, and it has not been disclosed that, as in the presentinvention, a large amount of blow air is introduced into a shaped bottleat least in the direction toward the bottom to thereby instantly coolthe lower part of the bottle.

On the other hand, in the present invention, a blow nozzle of a specificstructure as shown in FIG. 4 or 5 is used. When a blowing inlet orinlets of this nozzle are located at a position above about the centerof the parison and a blow air is introduced in a right downwarddirection (in the case of holding the parison with its bottom directeddownward), the blow air uniformly circulates along the inside surface ofthe bottle wall after reaching the bottom and, when a blow air isintroduced in an obliquely downward direction, it travels downward alongthe opposite inside surface of the bottle wall while cooling the surfaceand, after reaching the bottom, travels upward along the other insidesurface. Thus, the blow air circulates inside the bottle to effectivelycool the molding.

A blow nozzle generally designated by numeral 1 in FIGS. 4 and 5, whichis one embodiment of the present invention, has a first feature that,though front cross-section of the blowing inlets is almost circular,piston 9 sliding inside the body of the blow cylinder is of the form ofalmost ellipse in front cross-section with the major axis being in ahorizontal direction. This elliptical portion functions to resister thenozzle so that at least one of blowing inlets 2 is always positioneddownward. In this embodiment, four blowing inlets 2 are formed at theslight rear position 5 of the tip 3 of blow nozzle 1 in horizontal andvertical four directions almost at a right angle with each other. Thepassageway of the blow nozzle 1 in the rear of the blowing inlets 2forms a hollow portion 6, through which blow air is introduced and whichalso functions as a passageway for discharging blow air after completionof the molding.

The blow nozzle 1 shown in FIG. 5 has four blowing inlets capable ofblowing air in the upward, downward, leftward and rightward directions,respectively. Of the four blowing inlets, blowing inlet 2a capable ofblowing air in the downward direction has a larger cross-section thanthat of other blowing inlets 2b, 2c and 2d and allows air to be blowntherethrough in an amount of at least 30%, preferably at least 50%, morepreferably at least 70%, of the whole blow air amount. The blowing angle(θ₁) of the blowing inlet 2a is designed to be 10 to 20 degrees,preferably 13 to 17 degrees, particularly preferably about 15 degrees,which ensures effective circulation of the blown air inside the bottle.

The angle (θ₂) of the tip of nozzle is preferably 55 to 65 degrees, withabout 60 degrees being most preferable. Further, the slope angle (θ₃) offrom the thin-diameter portion of the nozzle tip to the blowing inlets 2is preferably 15 to 25 degrees, with about 20 degrees being mostpreferred.

Therefore, it suffices to form blowing inlets 2 in the blow nozzle 1 ata position slightly at the rear of the tip 3, at which the diameter islarger than that of the tip 3 and smaller than the outer diameter of thecentral portion 4, and at which the blowing inlets 2 can blow air at theabove-described direction so as to effectively circulate downward.

That is, in the present invention, the aforementioned blow nozzle 1 witha specific form pierces the upper wall of a parison, and blowing inlets2 are located at about the center of the parison, thus a large amount ofblow air being blown through the blow nozzle 1 against the inside wallof the bottle or in the right downward direction. When this blow air isintroduced into the bottle, the parison in a softened state is instantlyshaped into a bottle in conformity with the form of the metal mold and,at the point, the blow air migrates upward within the bottle to thin andbreak part of the bottle in the vicinity of or above the nozzle-piercedportion 7 shown in FIG. 2 and form an opening 8, through which part ofthe blow air is discharged out of the bottle. In addition, the opening 8serves for the blow air to uniformly circulate in a short time, thus theinside wall of the bottle being rapidly cooled.

However, this discharge opening is not designed to discharge the wholeblow air introduced through the blow nozzle 1 but to discharge only partof the blow air, and most of the air within the bottle is dischargedthrough the hole formed, after completion of the blowing, by rapidlymoving backward the blow nozzle 1, then through a discharge outlet 13formed in the vicinity of further backward moved blow nozzle andcommunicating to the outside of the body. That is, a large hole formedin the wall by the backward movement of the blow nozzle 1 functions as adischarge outlet for the inside air, and hence inside pressure of thebottle is instantly reduced to the atmospheric pressure.

Therefore, a blow nozzle having a large diameter can form alarge-diameter discharge outlet, which serves to shorten the timenecessary for discharging the inside air.

Structure of a blow air cylinder in accordance with the presentinvention is described below.

As is shown in FIGS. 6 to 10, the blow air cylinder of the presentinvention is constituted by a blow nozzle 1 which can freely slideforward and backward in the body 11, which has a mechanism ofpositioning blowing inlets at least at part of the nozzle, which has aclosed tip portion 3 having a diameter smaller than the outer diameterof the central portion 4, and which has blowing inlets 2 at the slightrear of the tip portion 3 and at a position having a diameter largerthan that of the tip portion 3 and smaller than the outer diameter ofthe central portion 4, said blowing inlets 2 enabling one to introduceblow air in the downward direction in an amount of at least 30% of thewhole blow air amount; and a body 11 disposed outside said blow nozzle 1in a state of being at least partly in contact with the nozzle.

In the wall in the vicinity of the tip of the body 11 is formed adischarge outlet 13 through which the blow air filling the metal mold iseffectively discharged upon the blow nozzle 1 being moved backward. Inthe outer wall of the body 11 are formed a blow air-feeding inlet 11afor moving forward the blow nozzle and a blow air-feeding inlet 12a formoving backward the blow nozzle. The blow nozzle 1 having a piston 9being freely moved forward and backward by the pressure of blow air isprovided inside the body 11.

At least part of the blow nozzle 1 has an about elliptical cross-sectionwith the major axis being horizontal. In FIG. 5, piston 9 has suchstructure. That is, the blow nozzle 1 does not have a circularcross-section over the full length, but at least part of the nozzle 1has an about ellipticaal cross-section with the major axis beinghorizontal, and hence the blow nozzle 1 is prohibited to rotate and,therefore, blowing inlets 2 of the nozzle can be directed always indefinite directions, thus functioning as a mechanism of positioning theblow nozzle 1.

The way how to work the blow air cylinder of the present invention isdescribed below by reference to cross-sectional views of one embodimentof the cylinder. FIG. 6 shows a blow air cylinder before work, FIG. 7the blow air cylinder in a state wherein introduction of blow air formoving forward a blow nozzle is initiated, FIG. 8 the blow air cylinderin a state wherein the blow nozzle has been moved forward by thepressure of blow air, FIG. 9 the blow air cylinder in a state whereinintroduction of blow air for moving backward the blow nozzle, and FIG.10 the blow air cylinder in a state wherein the blow nozzle has beenrestored to its original position by the pressure of air.

In FIGS. 6 through 10, air flow is shown by dotted band. Numeral 11designates a body, and 1 a blow nozzle having a structure capable ofbeing moved backward and forward inside the blow air cylinder and havinga piston.

In the outer wall of the body 11 are formed air-feeding inlet 11a formoving foreward the blow nozzle and air-feeding inlet 12a for movingbackward the blow nozzle after completion of the blow-molding. Theair-feeding inlet 11a is connected to the inside of the body 11 at aportion 11b at the slight rear of the center of the body 11, whereas theair-feeding inlet 12a is connected to the inside of the cylinder at atop portion 12b of the hollow portion 14 of the body.

In FIG. 7, the air blown through the air-feeding inlet 11a pressespiston 9 capable of being moved forward and backward in contact with theinside wall of the body 11 to thereby move forward the blow nozzle. Atthe point when the piston 9 reaches the top end of the hollow portion ofthe body 11, blow air-feeding inlet 21 formed at about the end of thenozzle 2 is laid bare in the hollow portion of the body 11, and thesubsequently introduced blow air travels through the blow air-feedinginlet 21, hollow portion 6, and the air-blowing inlets 2a, 2b, . . .formed at a slight rear of the tip portion 3 of the blow nozzle, finallyinto the inside of the blow molding (See FIG. 5).

Upon completion of the blowing, an automatic changing apparatus (notshown)) provided in the air-feeding equipment is actuated, and nowintroduction of air through the air-feeding inlet 12a is initiated asshown in FIG. 9. At this point, air-feeding inlet 12b is in the state ofconnecting to the space enclosed with the piston 9, the blow nozzle 1and the body 11, and the air fed through the air-feeding inlet 12a isintroduced into the inside of the body 11 through the air-feeding inlet12b, thus the piston 9 being pressed in the backward direction till theblow nozzle 1 is restored to its original position (FIG. 10).

Upon the blow nozzle is moved backward to leave the parison, the blowair filling the cavity of the metal mold is vigorously dischargedthrough the largish hole in the wall of the parison having been formedby the blow nozzle 1, then through a discharging hole 13 formed in thewall of the body 11 at a position in the vicinity of the top of the body11.

Part of the blow air can also be discharged through the blowing inlets2a, 2b, . . . formed in the blow nozle 1, migrates backward through thehollow portion 6 of the blow nozzle as well as through the aforesaiddischarge outlet 13.

In the blow molding method of the present invention, the use of suchspecific blow nozzle enables one to instantly blow a large amount ofblow air having been fed through the air-blowng inlet in the downwarddirection and, after completion of the molding, instantly dischargeoutside the blow air filling the cavity of the metal mold.

According to the present invention, blow air can be effectivelycirculated in the bottle and, since the hole formed after the blownozzle being moved backward and the discharge outlet 13 formed in thewall of the body in the vicinity of the tip of the body function asdischarge holes to instantly discharge a large amount of blow air out ofthe body, the blow molding cycle is markedly shortened. This advantagecan be obtained only by the combination of formation of an opening inthe vicinity of or above the blow nozzle-pierced portion of a parison bythinning and breaking the portion by the blow air or by piercing with aneedle-like member and the blow air cylinder having the above-describedspecific structure.

EXAMPLE

Advantages of the present invention are now described in more detail byreference to the following example.

Additionally, conditions under which the average temperature inthicknesswise direction becomes 108° C. when blow molding conditions of26° C. in blow air temperature, 15° C. in the surface temperature of themetal mold, and 13 seconds in cooling time are employed as in theconventional blow molding are taken as standard conditions.

Experiments of molding the same shaped moldings were conducted using thesame materials and changing part of the blowing conditions to determineconditions for cooling the moldings to the average temperature of 108°C. in the thicknesswise direction.

The blow nozzle used in the experiments of the present inventiondescribed in (3) and (4) below has four blowing inlets facing upward,downward, leftward and rightward, respectively, with the blowing inletfacing downward having a diameter of 3.5φ and the blowing inlets facingdownward, leftward and rightward having a diameter of 1.5φ, and apassageway having a diameter of 5φ and a cross-sectional area of 19.6mm², with the inlets having an angle of 15 degrees. This blow nozzle isso constituted as to blow 64% of blow air based on the whole blow airthrough the inlet facing downward.

(1) When blow molding was conducted under the conditions of 26° C. inblow air temperature and 5° C. in the surface temperature of the metalmold, the average temperature became 108° C. in 12.5 seconds. Therefore,the molding cycle was shortened by 5% by changing the surfacetemperature of the metal mold from 15° C. to 5° C.

(2) When blow molding was conducted under the conditions of 0° C. inblow air temperature and 15° C. in the surface temperature of the metalmol, the average temperature became 108° C. in 12.7 seconds. Therefore,the molding cycle was shortened by 3% by changing the blow airtemperature from 26° C. to 0° C.

(3) When blow molding was conducted under the conditions of 26° C. inblow air temerature and 15° C. in the surface temperature of the metalmold using the blow air cylinder of the present invention and piercingthe parison with the blow nozzle as shown in FIG. 2, the averagetemperature became 108° C. in 10.5 seconds. Therefore, the molding cyclewas shortened by 25% in this case.

(4) When the nozzle was moved backward immediately after blowing theblow air for about 9 seconds to form a hole in the blow molding (3),said hole functioning as a discharge hole for discharging the air insidethe bottle, the average temperature became 108° C. in 9.4 seconds.Therefore, the molding cycle was shortened by 40% in this case.

Additionally, in the above-described (1), it requires to feed a coolingwater of 0° C. or lower than that in order to keep the surfacetemperature of the metal mold at 5° C., which is industrially difficultand involves the problem of dropwise condeensation on the surface ofmetal mold, thus being impossible to practice. In the above-described(2), it is industrially difficult to feed blow air of 0° C. In addition,both (1) and (2) scarcely contribute to shortening of the blow-moldingtime. Thus, it can be seen that such blow molding conditions cannot bepractically employed.

As can be seen from the above-described results on the experiments, blowmolding cycle can be markedly shortened by instantly introducing blownair particularly in the downward direction to rapidly circulatethroughout inside the molding and by rapidly discharge the blow airintroduced in a pressed state.

Therefore, the structure of the nozzle realizing the effectivecirculation of the blow air inside the moldng has a great significance.

Industrial utility

As has been described in detail, according to the present invention, ablow air cylinder of a specific structure is employed, an opening isformed above the blow nozzle-pierced portion by the pressure of blow airor by piercing with a needle-like member from outside, and an upperportion of the parison is pierced with said blow nozzle to locate it atabout the center of the parison, thus effective circulation of the blowair being conducted on the inside surface of the molding. In addition, ahole formed after the blow nozzle being moved backward and a dischargehole in the vicinity of the tip of the body function as air-dischargingoutlets, which serve to instantly discharge the pressed air inside themolding. As a result, cooling time for the blow molding products ismarkedly shortened, which serves to shorten molding cycle and improvequality of the product such as dimensional accuracy, uniformity ofthickness, etc.

What is claimed is:
 1. A blow molding method for forming a hollowcontainer having a neck comprising the steps of:extruding a parison andintroducing the parison into a blow mold; closing the blow mold aboutthe parison and thereby pinching off the parison to form a top portionand a bottom portion: moving a blow nozzle to pierce the parison above alocation which is to form the neck of the container; introducing blowair through said blow nozzle to blow mold the parison and therebyshaping the parison into the hollow container; and forming an opening inthe blow molded parison at a position in the vicinity of the top portionof the parison to discharge the blow air and circulating the blow airwithin the blow molded parison to cool the hollow container; whereinsaid blow nozzle directs more of the blow air in the direction towardthe bottom portion of the parison than in other directions so that thecooling effect of the blow air is focused on the bottom portion, atleast 30% of the whole blow air amount being directed toward the bottomportion.
 2. The blow molding method as described in claim 1, whereinsaid opening is formed by thinning part of the wall of the parison byblow air until the wall of the parison breaks.
 3. The blow moldingmethod as described in claim 1, wherein said opening is formed bypiercing part of the wall of the parison with a needle-like member fromoutside of the parison.
 4. The blow molding method as described in claim1, wherein said blow nozzle further comprises a tip having a dischargeoutlet, and further comprising an outlet in the hollow container whereinthe blow air filling the parison is rapidly discharged through saidoutlet disposed in the vicinity of a tip of the hollow container bymoving said blow nozzle backward after completion of the blow molding.5. The blow molding method as described in claim 1, wherein blowinginlets are formed in the vicinity of the tip of the blow nozzle so thatthe blow air is introduced at least in a direction toward the bottom ofthe parison, which is opposite to the neck portion of the parison. 6.The blow molding method as described in one of claims 1 to 5, whereinintroduction of the blow air is continued after formation of theopening.
 7. The blow molding method as described in claim 1, whereinblowing inlets in the nozzle are formed in the upward and downwarddirection, with the amount of blow air introduced in the directiontoward the bottom being greater.
 8. The blow molding method as describedin claim 1, wherein blowing inlets in the nozzle are formed in theupward, downward, rightward and leftward directions, with the amount ofblow air introduced in the direction toward the bottom being greaterthan that in each of the other three directions.
 9. The blow moldingmethod as described in claim 1, wherein the amount of blow airintroduced in the direction toward the bottom is at least 50% of thewhole blow air amount.